The invention relates to radio communication within the high frequency band of 400 Hz to 2.7 KHz and in particular to modems for transmitting and receiving radio frequency signals.
High frequency (HF) communication performance can be degraded by three principle causes. Firstly propagation conditions can vary according to the time and frequency of transmission and also according to the geographical locations of the transmitter and receiver. Secondly, frequency selective fading and intersymbol interference can occur across the HF band because of ionospheric induced variations in multipath propagation. This arises as a result of ionospheric reflection by means of which high frequency transmission beyond the line-of-sight is achieved. More than one ionospheric reflective mode can exist and this causes multipath reception. Severe distortion can be observed on the transmitted data and in-band noise or interference which would normally be disregarded becomes the predominant factor during periods of signal fading. These conditions can only be completely avoided by frequency changing but this is usually impracticable. Undesirable effects of multipath propagation can be counteracted by employing appropriate signal processing before and after transmission. This is normally done using measurement of signal (spectral) spreading. The third cause of loss of performance is interference. Most of this encountered in the HF band is narrow band interference and can be avoided by changing the transmitting frequency. This is done by changing to an alternative channel frequency although where in-band frequency agility of the signalling tones can be used channel changing is not necessary. Wideband interference will always be a nuisance since it can only be avoided by frequency changing to a clear channel. When the wideband interference is impulsive, however, it can be avoided by adopting time dispersive coding into the transmitted signal format.